Eevee material specular
Reflexions depend on your point of view. If you want them not do do so, the best solution, for now, is not to use them, and rather fake their appearance.

One way to do this in 2.8 is to use the "Shader to RGB" node:



The principle is quite simple: from a diffuse shader, you get the shadow information created by the scene's lighting. And you convert that information to RGB data which you can then modify that data as you wish.

Of course, you can use other data as inputs, like the specularity of a glossy shader, an absolute ramp of a normal node, ... And you can combine them as you wish, as well as mix them with textures, ...

It's the basics of "NPR" (Non-Photorealistic Rendering, you can google those letters and find quite some examples of what people did.